Electronic Supplementary Information (ESI)

Covalently-capped seed-mediated growth: a unique approach towards hierarchical growth of gold nanocrystals

Youju Huang,# Anirban Dandapat# and Dong-Hwan Kim

School of Chemical and Biomedical Engineering, Nanyang Technological University, 637457,

Singapore

MATERIALS AND METHODS# These authors have contributed equally to this work.

Corresponding author: e-mail: dhkim@ntu.edu.sg; Tel: 65-67904111; Fax: 65-67911761

(DH. Kim)

Electronic Supplementary Material (ESI) for Nanoscale.This journal is © The Royal Society of Chemistry 2014

Chemicals

Cetyltrimethylammonium bromide (CTAB), 6-Mercaptohexanol (MCH), 11-

Mercaptoundecanol (MCU), 1-Nonanethiol (1-NT), 11-Mercaptoundecanoic acid

(MUA), 3-Mercaptopropionic acid (MPA), Cysteamine, Methoxy-poly(ethylene

glycol)-thiol (PEG-SH), hydrogen tetrachloroaurate (III) trihydrate (HAuCl4.3H2O),

ascorbic acid (AA), sodium borohydride (NaBH4) and silver nitrate (AgNO3) were

purchased from Sigma–Aldrich. All reagents were used as received.

Preparation of spherical Au seeds

The Au-seeds were prepared according to the Frens method.1 Briefly, 100 mL of

2.5×10–4 M HAuCl4 solution was heated to 120℃ in oil bath under vigorous stirring

for 30 min. Subsequently, 1% sodium citrate solution (1 ml and 5 ml for 40 nm and

18 nm particles, respectively) was added into above solution with continuous boiling.

After 20 min, the color of the boiled solution changed into ruby red, indicating the

formation of Au NPs in the solution.

Preparation of cubic Au seeds

Cubic gold NPs were prepared using a seeded growth method reported by Sau and

Murphy with slight modification.2 Briefly, the seeds were synthesized by the addition

of a freshly prepared, ice-cold aqueous NaBH4 solution (0.01 M, 0.6 mL) into an

aqueous mixture composed of HAuCl4 (0.01 M, 0.25 mL) and CTAB (0.1 M, 7.5 mL),

followed by rapid inversion mixing for 2 min. The resultant seed solution was kept at

room temperature for 1 h before use. The nanocubes were prepared by two step

growth process. The growth solution in the first step was prepared by the sequential

addition of CTAB (0.1 M, 6.4 mL), HAuCl4 (0.01 M, 0.8 mL), and ascorbic acid (0.1

M, 3.8 mL) into water (32 mL). 0.02 mL of the seed solution (diluted 10 times) was

then added into the growth solution. The resultant solution was mixed by gentle

inversion for 10 s and then left undisturbed overnight. The growth solution in the

second step was prepared by mixing 2.5×10-4 M HAuCl4, 0.01 M CTAB and 5.5 ×10-

4 Ascorbic Acid (AA) in 10 mL of water, then 3 mL of AuNPs solution in the first

step was added in the mixture, and kept at 30 C in water bath for at least 6 hours.

MCH, MCU, 1-NT, Cysteamine and PEG-SH modified Au-seeds

Au seeds were modified with –SH containing capping agents (MCH or MCU or 1-NT)

by replacing non-covalently bonded capping agents (e.g. citrate or CTAB). In brief,

20 mL of the original Au NPs (spherical or cubic seeds) were centrifuged (8000g, 20

min). The recovered nanocrystals were dispersed in DMF (20 mL). Then a solution of

thiolated ligands (100 mg) in DMF (2 mL) was added into 20 mL of Au NPs

(spherical or cubic seeds) in DMF maintaining a concentration of the ligands as 34

mM. After the solution was stirred for 24h, nanocrystals were recovered by

centrifugation (8000g, 20 min) followed by redispersion in water for further use.

MUA and MPA modified Au-seeds

20 ml as-prepared Au NPs were centrifuged (8000g, 20 min) to remove the excess

sodium citrate. After the supernatant had been removed, the precipitate was re-

dispersed in ethanol. During the purification process, a small amount of water

remained (∼5%, v/v). Then 2 ml ethanolic solution (5mM) of MUA (or MPA) was

added into the ethanolic suspension of Au NPs and left overnight.

Growth of Au nanostructures using different Au seeds

The growth solution was prepared by mixing 2.5×10-4 M HAuCl4, 0.01 M CTAB and

5.5×10-4 M AA. For the growth of different Au-seeds, 10 ml of growth solution was

mixed with a fixed amount of Au-seed solution in a plastic tube by gentle inversion

for 3 s and kept at 30 C in water bath for 6 hours. .

Characterizations

Field-emission scanning electron microscopy (FE-SEM) was performed on the JEOL

instrument (JSM-6700F) at an acceleration voltage of 5 kV and a working distance

between 7 and 8 mm. Raman spectra were collected using a Renishaw inVia confocal

Raman spectrometer with 50× objective lens in the range of 500−2000 cm−1 with one

accumulation and 10 s exposure time. A 632.8 nm wavelength HeNe laser (0.085 mW

at the sample surface) was used to excite the sample.

Figures

cba

Fig. S1 SEM images of (a) as prepared 18 nm Au NPs synthesized from citrate

reduction and (b,c) the nanostructures obtained after modification by MCH and MUA,

respectively. The scale bars in (a) and (b) represent 250 nm whereas those in the

insets and (c) are 50 nm.

a b

Fig. S2 SEM images of hierarchical Au nanostructures obtained from the growth of (a)

5 mM and (b) 70 mM MCH modified 18 nm spherical seeds. The scale bars represent

1 m.

ba

Fig. S3 SEM images of hierarchical Au nanostructures obtained from the growth of

MCH modified 40 nm cubic Au seeds. The scale bars in the images are 2 m and 200

nm, respectively.

a b

Fig. S4 SEM images of Au nanostructures obtained from the growth of non-

covalently modified 18 nm spherical (a) and 40 nm cubic seeds (b). The shapes of

seeds are maintained throughout the growth process. The scale bars represent 100 nm.

a b

c d

Fig. S5. SEM images of hierarchical Au nanostructures obtained from the growth of

mercaptoundecanol (a), 1- nonanethiol (b), cysteamine (c) and PEG-SH (d) modified

spherical Au seeds. The scale bars in the figures and insets are 1 m and 100 nm,

respectively.

ba

Fig. S6 SEM images of hierarchical Au nanowires obtained from growth of MUA

modified 18 nm Au seeds. The scale bars are 1m and 200 nm, respectively

ba

Fig. S7 SEM images of nanostructures obtained from the growth of (a) 1mM and (b) 20 mM of MUA modified 40 nm Au seeds. Inset of (b) shows some agglomerated spherical seeds were present as impurities along with the nanowires. The scale bars in the figures and inset are 1 m and 100 nm, respectively.

a b

Fig. S8 SEM images of hierarchical Au nanowires obtained from growth of 3-

mercaptopropionic acid-modified 40 nm Au seeds. The scale bars are 1m and 200

nm, respectively

Apparent Enhancement Factors (AEF)= ISERS [CNRS] / INRS [CSERS]

where CNRS is the concentration of R6G deposited on bare silicon substrate, CSERS is

the concentration of the adsorbed molecules on the surface of Au nanostructures, and

INRS and ISERS are the peak intensities corresponding to the peak at 1510 cm1 in

normal Raman spectra and SERS spectra measured from baseline, respectively.

References

1. G. Frens, Nature Phys. Sci., 1973, 241, 20-22.

2. T. K. Sau and C. J. Murphy, J. Am. Chem. Soc., 2004, 126, 8648-8649.